The present invention relates to a compact zoom lens with three lens groups and a design method thereof, especially to a compact zoom lens with three lens groups having about 3× zoom ratio and minimum length change of the lens while changing from a wide angle (WIDE) position to a telephoto (TELE) position.
In recent years, compact zoom lens with 3× zoom ratio has been well developed. After the problems of aberration correction have been solved, such zoom lens has been broadly applied to various cameras, digital cameras and video equipments. Among the compact zoom lens, the zoom lens with three lens groups has received quite a lot attention due to their features of compact size, high image quality and low aberration.
Users expect the zoom lens with features of high speed focusing, compact size and light weight while suppliers focus on easy driving and operation, precision focusing and low aberration of the lens as well as lower manufacturing cost. The focusing way of the zoom lens with three lens groups is divided into inner focus type and rear focus type. Refer to the rear focus type, similar devices are revealed by Japan patents JP2003-222797, JP2003-140041, JP2003-107348, U.S. Pat. No. 7,177,100, U.S. Pat. No. 7,164,539, U.S. Pat. No. 7,133,215, U.S. Pat. No. 7,031,073, U.S. Pat. No. 7,019,911, U.S. Pat. No. 6,943,960, U.S. Pat. No. 6,982,834, Taiwan patents No. I264558 and No. I259910. As to the inner focus type, refer to JP2006-209122, JP2006-184880, JP2004-109559, U.S. Pat. No. 7,136,232, US2006/0139767, U.S. Pat. No. 7,102,830, Taiwan patents No. I240801 and No. I251714. Moreover, in Japan patents 2004-380436, JP 2003-107348, a design of high angle shooting in WIDE position is disclosed.
In order to make the lens more compact, in prior arts, the first lens group includes at least one lens. While in rear focus, the first lens group moves far away form the image side in the TELE position while it comes close to the image side in the WIDE position. That means when changes from the WIDE position to the TELE position, distance D from an object side surface of the first lens of the first lens group to the image forming plane increases. Thus the whole zoom lens stretches out the camera. While moving or casing the camera, the zoom lens is easily to get impacted or damaged. This leads to inconvenience of users or reduced precision of the lens. In order to solve such problem, an actuator is added so as to move the zoom lens back into the camera. Thus the manufacturing cost of the camera is increased due to addition of the actuator.
As to the inner focus type, while in the TELE position or WIDE position, the focal length is changed by the movement of the second lens group, the third lens group, or the image forming plane so as to achieve zooming effect. However, while designing the zoom lens, positive or negative power, curvature, and relative distance among each lens of the first lens group, the second lens group, and the third lens group require to be compromised with one another. Even the first lens group is fixed, the fact that under better optical design of the zoom lens, the D (DW) in the WIDE position should be different from the D (DT) in the TELE position. The difference between the WIDE position D (DW) and the TELE position D (DT) will have effects on aberrations, astigmatism and distortion of the zoom lens. In U.S. Pat. No. 7,019,911, in order to resolve such problem, it is revealed that the difference (DW−DT) should be restricted within a certain range (0.4<(DW−DT)/f<1.2). Although such kind of limit can improve aberrations, astigmatism and distortion of the image in some degrees, basically the problem is not actually resolved. In view of designing, the present invention provides a method that makes the distance difference approach the minimum so as to get high speed focusing and good aberration correction either in the WIDE position or in the TELE position.